The sealing performance of a gasket depends not only on the quality and specification of the material, but on the flange design, surface finish, bolting material, thread lubrication and assembly methods. To obtain optimum performance from a gasket these external parameters must be taken into account.
Flange surface finish should not be left to the flange supplier but should be specified according to the type of gasket being used. Geegraf laminates can accommodate a wide range of surface roughness since the material readily flows into the flange grooves and imperfections under low bolt loads. A turned surface finish of 1.6 to 12.5 micro metres Ra is acceptable for most applications but for optimum performance a surface finish of 3.2 to 6.3 micro metres Ra is preferred.
Bolting material should be selected according to flange materials and working conditions and for general applications in the Petrochemical Industry alloys steel bolting Grade B7 and B16 are used. Typical specifications for these materials together with operating temperatures are given below:
It is important that bolt threads and nut faces are well lubricated prior to assembly. This is a simple operation that is often ignored particularly onsite. When replacement gaskets are being fitted after inspection or shutdown and bolting is being reused after working at elevated temperatures. The type of lubrication should be selected to suit the working conditions and a high temperature Nickel Anti-Seize compound such as Geegraf NAS not only helps on assembly but also makes it easier to dismantle on next service shutdown.
Bolt Lubrication Graph
The graph illustrates the effect of lubricating the bolt threads and nut faces under three conditions on a strain gauged bolt:
• A clean dry thread with no lubrication
• As received condition from the manufacturer with slight oil film
• Lubricated with Geegraf NAS compound
At 120lb ft torque the lubricated bolt provides four times the load of a dry bolt. Taking into account that the effort applied by the man with the spanner is identical, the lubrication allows the effort applied to be converted into a greater load on the gasket.
Bolt Tightening Diagram
The tightening sequence for the bolts is important so as to evenly distribute the bolt loading to compress and uniformly load the gasket
surface. The graphs left show the distribution of load in an eight bolt flange assembly after a diametrical tightening sequence at 50, 100
and 200 lb ft.
Although the bolts have all been tightened to the same torque loading after the third sequence the load in bolts 1 – 4 is half that of bolts 5 – 8.
Bolt Tightening Graph 2
The load can be easily evened out as illustrated by the next graph with a fourth tightening sequence at the same torque setting but tightening adjacent bolts in turn in a clockwise or anti-clockwise direction.
Three diametrical sequences followed by a fourth, tightening adjacent bolts in turn will provide an evenly distributed bolt loading and better gasket sealing performance.
The operating temperature of the bolts should also be taken into account particularly at elevated temperature and this is often well below the line temperature of the pipework. Position and insulation however can reduce this difference and bolt stress relaxation in service can severely reduce the load on the gasket.
Bolt Stress Relaxation Graph A
Graph A - shows percentage of stress retained after 1,000 hours service.
Bolt Stress Relaxation Graph B
Graph B - shows percentage of stress retained after 10,000 hours service.
Bolt Stress Relaxation Graph C
Graph C – shows percentage of stress retained after 100,000 hours service.
Note that after 100,000 hours service at 4000C. No load is left in a B7 bolt and only 50% in a B16 bolt and at 5000C
this has reduced to 10%. Consider the effect that this would have on the gasket loading and this applies to your
application ensure that a retightening sequence is applied to the bolts